To investigate how the concentration of sodium thiosulphate as it is diluted with water and then added to hydrochloric acid, affects the rate of reaction.

Method: These are my step-to-step procedures.

1. Collect all apparatus. (Check apparatus)
2. Clearly mark an X on a piece of white paper, as a marker.
3. I then continued to measure out 35cm3 of sodium thiosulphate, 00cm3 of water and 5cm3 of hydrochloric acid. (They were all measured in separate measuring cylinders)
4. I then got a beaker and put it on the piece of white paper with the black cross on it.
5. I firstly added the water then the hydrochloric acid. The sodium thiosulphate was added last.
6. I then started the stopwatch, while someone else stood over the solution to observe the cross.
7. I then repeated this 3 times each with each respective concentration. (Check quantities used)

Apparatus:  1) conical flask

                      2) Beaker

                      3) Measuring cylinder

                      4) White paper with black cross

                      5) Safety goggles

                      6) Hydrochloric acid

                      7) Sodium thiosulphate

                           8) Distilled water

                      9) Stopwatch

Diagram: How the experiment is set out

To completely make this a fair test I completely rinsed out the apparatus used to hold any of the substances to remove any particles from previous experiments so that there are the right amount of particles and therefore the concentration. I also measured each value properly so the concentrations were right because, having more or less of any solution could affect my results. I made sure I started the timer exactly when the solutions were together. I also made sure someone always stayed on top of the beaker to observe when the cross-disappeared. I always made sure that I never moved the beaker a lot because I didn’t want any more collisions resulting in a faster rate of reaction. I always repeated my exercise 3 times, so I could get a more accurate mean average. I used the Internet a lot to research on rate of reaction, so I could extend my knowledge on this subject, giving better observations.

 

Quantities used:

   

Results:  (on graph paper)

Conclusions: From my graph I can see that most of my results were correct, although I did have an anomalous result. My results agree with my prediction because I predicted that the lower the concentration of sodium thiosulphate, and the higher the amount of water, the longer the reaction would take to occur. My graph shows this because it curves downwards, indicating that as one factor decreased, so did the other. This is because before two particles can react they must meet. In a low concentration, the particles will be few and widely spread. This means that the number of reactions will be limited because fewer particles will meet. At higher concentrations there are more particles and so they probability of them coming into contact with other particles is increased. I increased the amount of water and decreased the amount of thiosulphate, the reaction will take longer and my graph line will curve down. For this to fully make sense it is necessary to recap the collision theory briefly: For a reaction to occur particles have to collide with each other. Only a small percent result in a reaction. This is due to the energy barrier to overcome. Only particles with enough energy to overcome the barrier will react after colliding. The minimum energy that a particle must have to overcome the barrier is called the activation energy, or Ea. The size of this activation energy is different for different reactions. If the frequency of collisions is increased the rate of reaction will increase. But in this collisions were decreased so reaction times took longer.

Evaluation: My method was suitable enough for this investigation. I say this because my prediction was right, so in turn my method could not have been too bad. There is still something’s to be desired from it. Ways to improve it would have been to spend more time on preparing the experiments by planning more thoroughly. It would have been better if some pre-liminary could be done with different concentrations, so we could at least get a better estimation of what the outcome would be, and give better observations. My observations and measurements could have been better by me spending a bit more time on them; my measurements were not fully up to standards because I got 3 anomalous results. These were probably due to me rushing the measuring of each solution. We were set a certain time limit of one 50-minute lesson, to get each solution done at least twice so I might have rushed. Other reasons for these anomalous results could be that the eye could also have made mistakes in judging whether or not the cross-had actually completely disappeared. If the same person watching the reaction was working the timer, errors could occur in their co-ordination. Using a burette to measure. This would remove the measuring errors associated with measuring cylinders, as they are correct to 0.1 cm where measuring cylinders are only 0.4. Burettes are a far more accurate way of measuring the correct amounts.

 Use light sensors to detect when the cross is no longer visible. The experiment could be connected to a light sensor, and to a timer. These light sensors will detect when there is no light shining through the substance. This would automatically stop the timer and therefore make the experiment fairer and more accurate.

Doing each solution on separate occasions and repeating them more than three times giving me a better average could have extended my investigation. But these choices were out of my control. Altogether I am quite pleased with my investigation and my results.